SAP2000 result; effect of changing support conditions

[Rafat125]

Hi all, I have been modelling a truss bridge in SAP2000. after modelling and running the analysis i go for the design. but the problem is if I change the support conditions to hinge at left end and roller at right end (both the trusses) from alternately hinged and roller (diagonally same support)the stress ratio at the bottom chord rises from 0.82 to 0.935. And This is happening for vertical load (i.e dead + live). can anybody tell me why is this happening? I have submit the final design within 1 month and I am confused about the arrangement of the supports. Should there be any effect of changing the supports for the design for vertical loads? As Both roller and hinge are capable to resist vertical loads. Please give me some advice.

Regards--

Rafat125

 

[asixth]

Welcome Rafat,

Have you include temperature effects in this load case?

I would image the stress in the truss would be higher if you modelled the same fixity conditions at diagonal supports because this is restraining your trusses from any horizontal movement because they are linked by the roadway.

If you model a hinge on the left hand side and roller on the right hand side (the simple beam fixity) for both trusses of your bridge, this effectivley eleviates any built up stresses that result from temperature changes.

 

[Rafat125]

Thnx for an early reply.

No sir I haven't included any temperature load cases. This is a foot over bridge supported at the ends on Elastomers and anchored with stair beams. I have restrained it at the same ends (from horizontal movement) by anchoring it to the beams keeping the other ends free to move (on Elastomers). but this arrangement gives higher stress than restraining it diagonally and keeping other diagonal supports free.

Expecting ur further advice!

Regards--

Rafat125

 

[Lion06]

I am not sure what you mean by restraining it diagonally. That being said a diagonal does have a horizontal component, so the program thinks you are restricting horizontal movement which will give you higher stresses - that much makes sense.

 

[asixth]

There is some confusion about the term "diagonal restraint". Are you referring to:

1) Two trusses
Truss 1: Left-pinned; Right-roller
Truss 2: Left-roller: Right-pinned

2) Rotating the direction of your roller support.

As StructuralEIT indicates, inclining your roller will induce some horizontal restraint.

 

[Rafat125]

yes i am referring to no. 1)..

 

[hokie66]

Look at the top chord. The benefit of transferring some of the load through the other framing, thus decreasing the load on the bottom chord, should be reflected in an increase on the load in the top chord.

 

[EdR]

Assuming you only have vertical loads applied to each case, the most likely cause of this behavior is an error in the model......If you have exactly the same truss with only vertical loads the results from each case must be the same....(At least I can't think of any case right now that would be different....)

Ed.R.